JP2003004924A - Diffractive optical element and optical device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a diffractive optical element which has almost constant diffraction efficiency for a wide wavelength range and which can be easily manufactured. SOLUTION: A first region and a second region with uniform distribution are settled in a diffraction grating having a step-like cross-sectional form in a single period. The level difference in one period of the second region is twice as the level difference in one period in the first region and the designed wavelength in the second region is twice as the designed wavelength in the first region. The number of steps in one period of the second region is same as the number of steps in one period of the first region and the horizontal width in the step-like part is made identical for all steps so that the diffraction efficiency for the designed wavelengths in the first and second regions is optimized. By controlling the ratio of the total area of the first region to the total area of the second region, the diffraction efficiency in the whole wavelength range from the designed wavelength in the first region to the designed wavelength in the second region is controlled to almost constant.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a diffractive optical element for diffracting light by a diffraction grating and an optical device using the same, and more particularly to a diffractive optical element having a substantially constant diffraction efficiency over a wide wavelength range.

[0002]

2. Description of the Related Art Diffractive optical elements that diffract light by a diffraction grating are used in optical systems of various optical devices.
The diffractive optical element has various advantages, but the main ones are as follows. (1) Since the diffraction angle depends on the wavelength, it can be used for spectroscopy. (2) If the surface on which the diffraction grating is provided has a curved surface and is used as a lens, the element becomes thinner than an ordinary lens having the same power. (3) The chromatic aberration of the optical system using the lens can be corrected because the positive and negative polarities of the wavelength dispersion characteristic of the lens are reversed.

The height difference of the unevenness of the diffraction grating is determined according to the design wavelength, and the diffraction efficiency decreases as the used wavelength deviates from the design wavelength. Further, the diffraction grating has the highest diffraction efficiency in the case of the blazed type having an inclined surface between the highest part and the lowest part within one period. In the case of this ideal blazed diffraction grating, assuming that the design wavelength is λ ₀ and the used wavelength is λ, the first-order diffraction efficiency η is expressed by the following equation 1. η = [sin {π (1-λ ₀ / λ)} / π (1-λ ₀ / λ)] ² Equation 1

FIG. 10 shows the diffraction efficiency when the design wavelength is 550 nm. In the wavelength range of 400 to 800 nm, the diffraction efficiency has a value of 61.5% to 100%. As described above, when the diffractive optical element is used for light in a wide wavelength range, the diffraction efficiency changes greatly. When the diffraction efficiency greatly differs depending on the wavelength, a spectroscope that detects the intensity of light for each wavelength must correct the output according to the wavelength. Also, in the camera, the correction of chromatic aberration becomes incomplete,
Color unevenness occurs in the captured image.

Regarding this problem, Japanese Patent Laid-Open No. 11-16
No. 0512 proposes that the diffraction grating is divided into three regions, and the design wavelength of each region is set to 450, 532, and 614 nm, and the height difference is made different for each region. The diffraction efficiency in this setting is shown in FIG. The diffraction efficiency of each region is 100% at each design wavelength. However, in reality, not only light with a wavelength close to the design wavelength is incident on each region, but light with a wide wavelength range is incident on any region, so the overall diffraction efficiency of the diffraction grating as a whole. Is the average value of the diffraction efficiencies in the three regions. For example, when the areas of the three regions are equal, the total diffraction efficiency is 400 to 8
It is 66.3% to 95.0% in the wavelength range of 00 nm.
For this reason, the problem that the diffraction efficiency changes greatly cannot be solved.

The blazed diffraction grating is ideal from the viewpoint of diffraction efficiency, but it is difficult to manufacture and is not suitable for mass production. Therefore, the inclined surface between the highest portion and the lowest portion of the blazed type is approximated by a step-like shape. Such a diffraction grating is called a multilevel type. Since the multi-level type diffraction grating is composed of only the horizontal plane and the vertical plane, it can be easily manufactured by the photolithography established in the semiconductor technology. For example, in US Pat. No. 4,895,790, using k photomasks,
By performing the etching k times, the number of steps is (2 ^k -1)
It has been proposed to make a staircase shape. In the multi-level type, as the width of the horizontal plane is narrowed and the number of steps is increased, the diffraction efficiency becomes higher (closer to the diffraction efficiency of the ideal blaze type), but the width of the horizontal plane has the minimum limit in processing. The optimum number of stages is determined according to the period (pitch) of the diffraction grating.

[0007]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
Also in No. 60512, the diffraction grating is of a multilevel type, and in order to make the maximum height difference of the three regions different, the steps (height difference of adjacent horizontal planes) are made the same and the number of steps is made different for each region. In this method, since the number of stages is different for each region, if the optimum number of stages is determined on the basis of the region with the smallest maximum height difference, that is, the region with the smallest number of stages, the number of stages in other regions becomes larger than the optimal number of stages. It becomes necessary to make the width of the horizontal plane below the minimum limit. If the width of the horizontal plane is less than the minimum limit, the staircase shape is broken and the diffraction efficiency is reduced. Conversely, if the optimum number of steps is determined based on the area with the largest maximum height difference, that is, the area with the largest number of steps,
In other regions, the number of steps is smaller than the optimum number of steps, and it becomes impossible to obtain the diffraction efficiency that should be obtained with the optimum number of steps.

Further, the ratio of the maximum height difference between regions is the ratio of design wavelengths, and is not a simple positive number ratio. For this reason,
Except by chance, there are no horizontal planes with the same height or horizontal planes with the same height difference in different regions, and different regions cannot be processed by one etching. As a result, it becomes necessary to carry out etching individually for each region, resulting in poor manufacturing efficiency.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a diffractive optical element which has a substantially constant diffraction efficiency over a wide wavelength range and is easy to manufacture. Another object of the present invention is to provide a high-performance spectroscopic device and an imaging device equipped with such a diffractive optical element.

[0010]

In order to achieve the above object, according to the present invention, in a diffractive optical element having a step-like diffraction grating whose cross section of one period has equal steps, the diffraction grating is
The first area in which the number of steps in one cycle is constant and the maximum height difference in one cycle is constant, and the number of steps in one cycle is constant and is equal to the number of steps in one cycle of the first area It is assumed that the maximum height difference is constant and the second area is twice the maximum height difference in one cycle of the first area.

The diffraction grating of this diffractive optical element has two types of regions, and the maximum height difference of the second region is twice the maximum height difference of the first region. That is, the design wavelength of the first region and the second region is 1: 2, and the diffraction grating efficiently diffracts the light of the first wavelength and the light of the second wavelength having the wavelength ratio of 1: 2. be able to. Moreover, since the first region and the second region have the same number of steps, the width of the horizontal plane of the stepped portion is the same, and both can be set to give the highest diffraction efficiency according to the number of steps. It is possible. That is,
It is possible to equalize the diffraction efficiency of the light of the first wavelength by the first region and the diffraction efficiency of the light of the second wavelength by the second region.

The diffraction efficiency of the entire diffraction grating for light of an arbitrary wavelength is the diffraction efficiency of the first region and the diffraction efficiency of the second region with the area ratio of the first region and the second region as a weighting coefficient. Is a weighted average of. As is clear from Equation 1 and FIG. 10, the diffraction efficiency is asymmetric with respect to the design wavelength, but depending on the setting of the area ratio of the first and second regions, the entire wavelength range from the first wavelength to the second wavelength is It is possible to make the diffraction efficiency substantially constant over a wide wavelength range including a part of.

In the first area and the second area, the maximum height difference ratio is 1: 2 and the number of steps is the same, so the step ratio is also 1: 2. Will have a set of equal horizontal planes. When the diffraction grating is manufactured by photolithography, the horizontal plane having the same height or the lower horizontal plane having the same height difference can be simultaneously formed by one etching, so that the number of steps can be reduced. Therefore, the manufacturing efficiency is also improved.

Here, it is preferable that the first region and the second region are substantially uniformly distributed over the entire diffraction grating. Macroscopically, the first region and the second region are present at a constant area ratio in any part of the diffraction grating, and it is possible to avoid a difference in diffraction efficiency between the parts.

The wavelength of the highest diffraction efficiency in the first region when the incident angle is 0 ° is preferably 350 nm or more and 550 nm or less. In this case, the wavelength of the highest diffraction efficiency in the second region when the incident angle is 0 ° is 700 nm or more and 110
It becomes 0 nm or less. This makes it possible to diffract the entire visible light with a substantially constant diffraction efficiency in a usage pattern in which the incident angle does not greatly deviate from 0 °.

The total area of the first region may be 1.7 times or more and 2.6 times or less of the total area of the second region. When the area ratio of the first and second regions is set as described above, the degree of constant diffraction efficiency with respect to the entire visible light becomes particularly high.

Further, in the present invention, the spectroscopic device is provided with any one of the above diffractive optical elements in a part of an optical system for separating light according to a wavelength. The intensity of the separated light can be made substantially constant regardless of the wavelength, and it is not necessary to correct the intensity.

Further, in the present invention, the image pickup device is provided with any one of the above diffractive optical elements in a part of an optical system for forming an image of light. It is possible to satisfactorily correct the chromatic aberration caused by the refraction by the lens, and it is possible to avoid the occurrence of color unevenness in the image.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. The configuration of the diffractive optical element 1 according to the first embodiment is schematically shown in FIG. The diffractive optical element 1 is composed of a flat plate-shaped substrate 10 on the surface of which a diffraction grating 11 which is a periodic array of irregularities is formed. FIG. 1 is a plan view of the entire diffractive optical element 1, and does not show the shape itself of the diffraction grating 11 having an extremely fine structure. Diffraction grating 1
Reference numeral 1 has two types of regions 11a and 11b having different diffraction characteristics. Both the first region 11a and the second region 11b are strip-shaped and are present in a large number, and are provided alternately in the width direction (x direction).

A contour diagram of the diffraction grating 11 is schematically shown in FIG. In the diffraction grating 11, the direction (x direction) of the periodic array of the concavities and convexities matches the width direction of the regions 11a and 11b.
That is, the points having the same height are set to be continuous in the length direction (y direction) of the regions 11a and 11b.

FIG. 3 schematically shows a cross section of the diffraction grating 11 in the periodic direction. The diffraction grating 11 is a multi-level type in which a blaze-type inclined surface is approximated by a step shape, and has a plurality of horizontal planes within one period p. The period of the diffraction grating 11 is constant throughout, and the first region 11a and the second region 1
The cycle is the same in any of 1b. The start point and the end point of one cycle can be arbitrarily defined, but here, the period from the boundary between the highest horizontal plane and the lowest horizontal plane to the boundary between the highest horizontal plane and the lowest horizontal plane is referred to as one cycle p.

The maximum height difference (height difference between the highest horizontal plane and the lowest horizontal plane) h2 within one cycle of the second region 11b is
It is twice the maximum height difference h1 within one cycle of the first region 11a. On the other hand, the number of steps in one cycle (the number m-1 of horizontal planes) is the same in the first region 11a and the second region 11b, and the step (height difference between adjacent horizontal planes) is the same in one period. Therefore, the step d2 of the second region 11b is the first
Is twice as large as the step d1 in the area, and the widths w of the horizontal plane are all equal.

The diffraction grating 11 is one in which the height difference between the highest part and the lowest part of the inclined surface is divided into m equal parts, and each equal point and the highest part are made into a horizontal plane, which corresponds to the lowest part of the inclined surface. No horizontal plane exists. The example shown in FIG. 3 is a case where the height difference between the highest part and the lowest part of the inclined surface is divided into four equal parts, and therefore, the number m of horizontal surfaces in one cycle is 4, and the number of steps is 3. .

In the diffraction grating 11 having such a configuration, the second
The design wavelength of the area 11b is set to be twice the design wavelength of the first area 11a. Further, since the widths w of the horizontal plane are all the same, it is possible to set the width w of the horizontal plane to the minimum limit in processing in both the first region 11a and the second region 11b. The diffraction efficiency of the light of the design wavelength by the area 11a and the diffraction efficiency of the light of the design wavelength by the second area 11b can be equalized and maximized.

The total diffraction efficiency ηt of the diffraction grating 11 as a whole with respect to light of an arbitrary wavelength λ is determined by the first region 11a.
Is the weighted average of the diffraction efficiency η1 of the second region 11b and the diffraction efficiency η2 of the second region 11b, which is expressed by Equation 2. ηt = η1 · S1 / S + η2 · S2 / S ... Formula 2

Here, S1 and S included in the weighting factors
2 is the total area of the first region 11a and the total area of the second region 11b, and S is the total area (S1 + S2) of the diffraction grating 11. In addition, the first and second regions 11
The diffraction efficiencies η1 and η2 of a and 11b are calculated by the above-mentioned formula 1.

Design wavelengths λ ₀ of the first region 11a and the second region 11b are 440 nm and 880 nm, respectively, and the ratio of the total area S1 of the first region 11a and the total area S2 of the second region 11b is 2: When the total diffraction efficiency ηt of the diffraction grating 11 is set to 1, the diffraction efficiency η of each of the regions 11a and 11b is
1 and η2 are shown in FIG. The total diffraction efficiency ηt of the diffraction grating 11 in the wavelength range of 400 to 800 nm is 6
It is 4.8 to 68.4%, which is almost constant. When the design wavelength λ ₀ of the first region 11a and the second region 11b is set to these values, the total area S1 of the first region 11a is set to the second
If the range is 1.7 to 2.6 times the total area S2 of the region 11b, the ratio of the maximum value to the minimum value of the diffraction efficiency ηt in this wavelength range is 1.1 or less.

FIG. 4 shows the case where the number of steps in one cycle is infinite, that is, when the ideal blazed type is used, and the smaller the number of steps, the lower the diffraction efficiency. For example, the number of steps is 15, 7, 3 (the number of horizontal planes m is 16, 8,
In 4), the diffraction efficiency is 98.
It becomes 7,95.0,81.1%. However, the decrease in the diffraction efficiency due to the approximation of the stepped shape does not affect the fact that the diffraction efficiency between the two design wavelengths is substantially constant.

As will be described later, the diffraction grating 11 is produced by photolithography, but the minimum limit of the width w of the horizontal plane that can be formed by etching is about 1 μm. Further, when diffracting light in the visible wavelength range as described above, the period p of the diffraction grating 11 is about 4 μm, so
The optimum value of the number of steps in the cycle is 3 (the optimum value of the number m of horizontal planes is 4). Therefore, the diffraction grating 11 can diffract the entire visible light with a substantially constant diffraction efficiency of 50% or more.

Since the maximum height differences h1 and h2 within one cycle are one less than the number of steps of the height difference of the inclined surface divided by 1, the ideal blazed maximum height difference h (m- 1)
/ M times. When light is incident on an ideal blazed diffraction grating at an incident angle of 0 °, the maximum height difference h is expressed by Equation 3 when it is a transmission type and Equation 4 when it is a reflection type. Here, n is the refractive index of the medium, and in Equation 3, the refractive index of the substrate, in Equation 4, when the light is incident from the substrate side, the refractive index of the substrate, and when the light is directly incident from the air side, The refractive index of air, that is, 1. h = λ ₀ / (n−1) Equation 3 h = λ ₀ / 2n Equation 4

The design wavelength λ ₀ is ideally determined in consideration of the incident angle of light when the diffractive optical element 1 is actually used. However, in the diffractive optical element 1 of the present embodiment, since the diffraction efficiency can be made substantially constant over a wide wavelength range, it is not necessary to strictly consider the incident angle to determine the design wavelength λ ₀ . For example, the incident angle is set to 0 ° and the first region 11
The design wavelength of a is set to a value within the range of 350 to 550 nm,
If the diffraction grating 11 is prepared with the design wavelength of the second region 11b being twice as large as the design wavelength, the entire visible light is diffracted at a substantially constant diffraction efficiency even when the incident angle during use is about 30 °. Can be made.

A method of manufacturing the diffraction grating 11 will be described by taking as an example a case where the number of steps is 3, that is, the number m of horizontal planes is 4. The manufacturing process is schematically shown in FIG. First, a photoresist (not shown) is applied to the flat surface of the substrate 10, the photoresist is patterned using a mask M1 having an opening E1 having a width 1/2 of one period p, and etching is further performed (a ). The etching depth is four times the step d1 of the first region 11a. This forms a horizontal plane including the second horizontal plane from the bottom of the second region 11b.

Thereafter, the steps of applying photoresist, patterning using a mask and etching are repeated twice. In the second step (b), the opening E1 having a width of ½ of one cycle p and the two openings E2 having a width of ¼ of one cycle p.
Using the mask M2 having the above, the depth to be processed by etching is set to be twice the step d1 of 11a in the first region. With this, the horizontal plane including the second horizontal plane from the bottom of the first region 11a and the lowermost horizontal plane of the second region 11b and 3 from the bottom.
A second horizontal plane is formed at the same time.

In the third step (c), 1 / p of one cycle p
Using a mask M3 having two openings E2 having a width of 4 and a step depth d of 11a in the first region
It is 1 times 1. This forms the lowest horizontal plane and the third horizontal plane from the bottom of the first region 11a.
Preparation of 1 is completed.

Note that the above three steps may be performed in any order as long as the mask and the depth to be processed by etching correspond to each other. Only the order of forming the horizontal plane is different,
The manufactured diffraction gratings 11 have the same shape.

When the first region 11a and the second region 11b are separately manufactured, four masks are used and the number of steps is 4, but the above method reduces the number of masks required and the number of steps. The manufacturing efficiency is improved. This is the first area 1
This is because there are horizontal planes having the same height and horizontal planes having the same height difference in 1a and the second region 11b.

In general, it is possible to form (2 ^k -1) steps having a horizontal plane of 2 ^k by etching k times, but if the two regions are individually etched, the total number of steps is 2 k. . On the other hand, in the above method capable of simultaneously etching two regions, the total number of steps is (k +
1) is enough. Therefore, as the number of stages increases, the effect of improving the manufacturing efficiency becomes more remarkable. Note that the maximum number of steps that can be formed by (k + 1) times of etching is (2 ^k −1), and can be set to any value less than this.

The width of the first region 11a and the second region 11b is about several tens of times the period p of the diffraction grating 11, and
The second regions 11a and 11b are distributed substantially uniformly over the entire diffraction grating 11. Therefore, it is possible to diffract light in a wide wavelength range with substantially constant diffraction efficiency at any part of the diffraction grating 11. With this setting, even if the incident position of light changes, the area ratio of the first and second regions 11a and 11b to the light does not change, so that the diffraction efficiency is kept substantially constant over a wide wavelength range. Is certainly realized.

When the diffractive optical element 1 is used for light whose wavelength distribution is uniform at any part of the cross section of the light beam,
Even if the distributions of the first region 11a and the second region 11b are uneven, the diffraction efficiency as a whole is substantially constant. Therefore, in such an application, the first and second regions 11a,
It is not necessary for 11b to be distributed substantially uniformly over the entire diffraction grating 11. For example, the diffraction grating 11 may be divided into two sections, one of which is the first area 11a and the other is the second area 11b. it can.

However, in such a setting, the diffraction grating 1
The area ratio of the first region 11a to the second region 11b with respect to the light varies depending on which part of 1 the light enters, and it becomes difficult to make the diffraction efficiency constant.
This problem can be avoided by strictly determining and fixing the incident position of the light, but in order to do so, the diffractive optical element 1 and other optical elements for guiding the light to the diffractive optical element 1 must be provided. It is necessary to set the positional relationship with high accuracy, and it takes time to adjust. Therefore, the first region 11a and the second region 11a
It is preferable that the regions 11b of 1 are distributed substantially uniformly.

Here, the first regions 11a and the second regions 11b are alternately arranged in a straight band shape, but as long as the area ratio is determined according to the design wavelength based on the equation 2, the first regions 11a and 11b are alternately arranged. The shape and the positional relationship between 11a and the second area 11b can be set to other settings. An example is shown in FIG. In FIG. 6, (a) shows the second area 11b in a rectangular shape and is dispersed in the first area 11a. It was placed in. In the case of (b), the diffraction grating 11 is also a concentric circle. First and second areas 11a and 11b
Can also be arranged alternately as arcs that are not concentric circles. An example of the diffraction grating 11 in that case is shown in the contour diagram of FIG. 7.

The wavelength of the light to be diffracted may be outside the visible light range. For example, ultraviolet light in the wavelength range of 200 to 400 nm and infrared light in the wavelength range of 1000 to 2000 nm can be targeted.

The structure of the spectroscopic device 2 according to the second embodiment is schematically shown in FIG. The spectroscopic device 2 separates light according to the wavelength and individually detects the intensity of the separated light. The light flux regulating plate 21 having the slit 21a, the two concave mirrors 22 and 24, the diffractive optical element 23, and It comprises a sensor 25.

The light passing through the slit 21a is reflected by the concave mirror 22, enters the diffractive optical element 23 while converging, and is reflected at a diffraction angle according to the wavelength. The light reflected by the diffractive optical element 23 is reflected by the concave mirror 24 and enters the sensor 25 while further converging. When entering the sensor 25, the light is separated for each wavelength, and the sensor 25 detects the intensity of the light of each wavelength.

As the diffractive optical element 23, the diffractive optical element 1 of the first embodiment is used as a reflective type. Since the diffractive optical element 1 has a substantially constant diffraction efficiency for light in a wide wavelength range as described above, the intensity ratio of light of each wavelength detected by the sensor 25 faithfully represents the intensity ratio before separation. Therefore, it is not necessary to correct the output of the sensor 25.

When used as the diffractive optical element 23, the diffraction grating 11 is formed into the arc shape shown in FIG.
It is preferable to arrange so that the light from is incident along the periodic direction (x direction) of the diffraction grating 11. Concave mirrors 22 and 24
As a result, astigmatism occurs, but the astigmatism can be corrected.

The configuration of the image pickup apparatus 3 according to the third embodiment is schematically shown in FIG. The image pickup device 3 is for taking an image by converting light into an electric signal, and as an image pickup optical system for forming an image of light from an object to be imaged, two lens groups 31,
34, a diffractive optical element 32, and a diaphragm 33, and an area sensor 35 as an image pickup element on the image forming surface of the photographing optical system.

As the diffractive optical element 32, the diffractive optical element 1 of the first embodiment is used as a transmission type, but the substrate 10 has concave surfaces on both sides, and the diffractive optical element 32 also functions as a concave lens. The diffraction grating 11 is set in a concentric circle shape, and the first and second regions 11a and 11b are also in a concentric band shape as shown in FIG. 6B.

The lens groups 31, 3 are used for the light from the object to be photographed.
Refraction by the diffractive optical element 32 as 4 or a concave lens,
Chromatic aberration occurs, which can be corrected by diffraction by the diffractive optical element 32. Therefore, it is possible to provide a high-quality image with no color shift. The occurrence of chromatic aberration can be suppressed by increasing the number of lenses included in the lens groups 31 and 34. However, if the diffractive optical element 32 is provided, it is possible to correct chromatic aberration without increasing the number of lenses.

The embodiments of the diffractive optical element 1 and the spectroscopic device 2 and the image pickup device 3 using the diffractive optical element 1 have been described above. However, the diffractive optical element of the present invention is not limited to the examples shown here, and various optical devices can be used. Can be used for.
For example, even when used in a spectrocolorimeter, an optical pickup, an optical communication device, a laser beam printer, a copying machine, a microscope, etc., the characteristic that the diffraction efficiency is substantially constant for light in a wide wavelength range is utilized.

[0051]

The cross section of one cycle has a stepped diffraction grating having the same steps, and the diffraction grating has a constant number of steps in one cycle,
A first area having a constant maximum height difference in one cycle and a step number in one cycle equal to the number of steps in one cycle of the first area;
According to the diffractive optical element of the present invention, which has a constant maximum height difference within one cycle and is twice the maximum height difference within one cycle of the first area, the first embodiment has a wavelength ratio of 1: 2. It is possible to diffract both the light of the wavelength and the light of the second wavelength with the highest diffraction efficiency according to the number of steps of the diffraction grating. Also, the first and second
By determining the area ratio of the first and second regions according to the wavelength of, the diffraction efficiency can be made substantially constant over the entire wide wavelength range including the first wavelength to the second wavelength. Moreover, since the horizontal surface of a part of the first region and the horizontal surface of a part of the second region can be simultaneously formed by etching, the manufacturing efficiency is excellent.

In the structure in which the first region and the second region are substantially evenly distributed over the entire diffraction grating, the first region and the second region are macroscopically located at every portion of the diffraction grating. Since they exist with a constant area ratio, there is no difference in diffraction efficiency between the parts. Therefore, the degree of freedom of the incident position of light is increased.

When the wavelength of the highest diffraction efficiency in the first region when the incident angle is 0 ° is set to 350 nm or more and 550 nm or less, the entire visible light is diffracted to a substantially constant value unless the incident angle is greatly separated from 0 °. It can be diffracted with high efficiency and becomes a versatile element.

When the total area of the first region is 1.7 times or more and 2.6 times or less of the total area of the second region, the constant degree of diffraction efficiency for the entire visible light can be made particularly high. it can.

In the spectroscopic device of the present invention in which the diffractive optical element having the above-mentioned features is provided in a part of the optical system for separating the light according to the wavelength, the intensity of the separated light is substantially constant regardless of the wavelength. can do. Therefore, it becomes an easy-to-use spectroscopic device that does not need to correct the intensity according to the wavelength.

Further, in the image pickup apparatus of the present invention in which the diffractive optical element having the above-mentioned features is provided in a part of the optical system for forming an image of light, the chromatic aberration caused by the refraction by the lens is favorably corrected by the diffractive optical element. It is possible. Therefore, the image pickup apparatus can provide a high-quality image without color unevenness.

[Brief description of drawings]

FIG. 1 is a plan view showing a region of a diffraction grating of a diffractive optical element according to a first embodiment.

FIG. 2 is a contour diagram of a diffraction grating of the diffractive optical element.

FIG. 3 is a sectional view of a diffraction grating of the diffractive optical element.

FIG. 4 is a diagram showing the diffraction efficiency of the diffractive optical element.

FIG. 5 is a cross-sectional view showing a step of producing a diffraction grating of the diffractive optical element.

FIG. 6 is a plan view showing a modified example of a region of a diffraction grating of the diffractive optical element.

FIG. 7 is a contour diagram of a modified example of the diffraction grating of the diffractive optical element.

FIG. 8 is a diagram schematically showing a configuration of a spectroscopic device according to a second embodiment.

FIG. 9 is a diagram schematically showing a configuration of an image pickup apparatus according to a third embodiment.

FIG. 10 is a diagram showing the diffraction efficiency of a conventional diffractive optical element.

FIG. 11 is a diagram showing the diffraction efficiency of another conventional diffractive optical element.

[Explanation of symbols]

1 Diffractive optical element 10 substrates 11 diffraction grating 11a First area 11b Second area 2 Spectroscopic device 21 Luminous flux regulation plate 21a slit 22 concave mirror 23 Diffractive optical element 24 concave mirror 25 sensors 3 Imaging device 31 lens group 32 diffractive optical element 33 aperture 34 lens group 35 Image sensor

Claims (6)

[Claims] 1. A diffractive optical element having a step-like diffraction grating with a cross section of one cycle having equal steps, wherein the diffraction grating has a constant number of steps within one cycle and a constant maximum height difference within one cycle. And the number of steps in one cycle is constant and equal to the number of steps in one cycle of the first area, the maximum height difference in one cycle is constant and the maximum height difference in one cycle of the first area is two.
A diffractive optical element comprising a doubled second region. 2. The diffractive optical element according to claim 1, wherein the first region and the second region are substantially uniformly distributed over the entire diffraction grating. 3. The diffractive optical element according to claim 1, wherein the wavelength of the highest diffraction efficiency in the first region when the incident angle is 0 ° is 350 nm or more and 550 nm or less. . 4. The total area of the first region is 1.7 times or more and 2.6 times or less of the total area of the second region, according to any one of claims 1 to 3. The diffractive optical element according to the item. 5. A spectroscopic device comprising the diffractive optical element according to claim 1 in a part of an optical system that separates light according to a wavelength. 6. An image pickup apparatus comprising the diffractive optical element according to claim 1 in a part of an optical system for forming an image of light.